Just to confirm, my contract of employment is the original contract from when I first started - my salary was somewhere around £15k as it was a few years ago... however in the letter from my employer they do state my current salary which is greater than £18.6k, should this be enough?
That's probably fine.
Does the employer letter state what date you started earning over £18,600? It is a requirement that the letter states this, so that UKVI can confirm, along with your payslips and bank statements, that you have been earning more than £18,600 for at least 6 months.
The employer letter must be dated within 28 days of the online application date and state ALL of the following:
1) Current employment and salary
2) Length of employment
3) Length of time earning current salary
4) Type of employment (permanent, temporary, full/part-time etc.)